The present invention relates to the use of Code Division Multiple Access (CDMA) communications techniques in cellular radio telephone communication systems, and more particularly, to methods and systems related to power control in systems using discontinuous Direct Sequence-Code Division Multiple Access (DS-CDMA) transmissions.
DS-CDMA is one type of spread spectrum communication. Spread spectrum communications have been in existence since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications. Some examples include digital cellular radio, land mobile radio, satellite systems and indoor and outdoor personal communication networks referred to herein collectively as cellular systems.
Currently, channel access in cellular systems is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal""s transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which pass substantial signal energy only within the specified frequency band. Thus, with each channel being assigned a different frequency band, system capacity is limited by the number of available frequency bands as well as by limitations imposed by frequency reuse.
In TDMA systems which do not employ frequency hopping, a channel consists of a time slot in a periodic train of time intervals over the same frequency band. Each period of time slots is called a frame. A given signal""s energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that passes signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
With FDMA or TDMA systems (or hybrid FDMA/TDMA systems), one goal is to insure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) is an access technique which uses spread spectrum modulation to allow signals to overlap in both time and frequency. There are a number of potential advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be higher than that of existing analog technology as a result of the properties of wideband CDMA systems, such as improved interference diversity and voice activity gating.
In a direct sequence (DS) CDMA system the symbol stream to be transmitted (i.e., a symbol stream which has undergone channel encoding etc.) is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data (commonly referred to as xe2x80x9cchipsxe2x80x9d) are binary or quaternary, providing a chip stream which is generated at a rate which is commonly referred to as the xe2x80x9cchip ratexe2x80x9d. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The symbol stream and the signature sequence stream can be combined by multiplying the two streams together. This combination of the signature sequence stream with the symbol stream is called spreading the symbol stream signal. Each symbol stream or channel is typically allocated a unique spreading code. The ratio between the chip rate and the symbol rate is called the spreading ratio.
A plurality of spread signals modulate a radio frequency carrier, for example by quadrature phase shift keying (QPSK), and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique codes, and the corresponding signal can be isolated and decoded.
For future cellular systems, the use of hierarchical cell structures will prove valuable in even further increasing system capacity. In hierarchical cell structures, smaller cells or micro cells exist within a larger cell or macro cell. For instance, micro cell base stations can be placed at a lamp post level along urban streets to handle the increased traffic level in congested areas. Each micro cell might cover several blocks of a street or a tunnel, for instance while a macro cell might cover a 3-5 Km radius. Even in CDMA systems, it is likely that the different types of cells (macro and micro) will operate at different frequencies so as to increase the capacity of the overall system. See, H. Eriksson et al., xe2x80x9cMultiple Access Options For Cellular Based Personal Comm.,xe2x80x9d Proc. 43rd Vehic. Tech. Soc. Conf., Secaucus, 1993. Reliable handover procedures must be supported between the different cell types, and thus between different frequencies so that mobile stations which move between cells will have continued support of their connections.
There are several conventional techniques for determining which new code, frequency and cell should be selected among plural handover candidates. For example, the mobile station can aid in the determination of the best handover candidate (and associated new base station) to which communications are to be transferred. This process, typically referred to as mobile assisted handover (MAHO), involves the mobile station periodically (or on demand) making measurements on each of several candidate frequencies to help determine a best handover candidate based on some predetermined selection criteria (e.g., strongest received RSSI, best BER, etc.). In TDMA systems, for example, the mobile station can be directed to scan a list of candidate frequencies during idle time slot(s), so that the system will determine a reliable handover candidate if the signal quality on its current link degrades beneath a predetermined quality threshold.
In conventional CDMA systems, however, the mobile station is continuously occupied with receiving information from the network. In fact, CDMA mobile stations normally continuously receive and transmit in both uplink and downlink directions. Unlike TDMA, there are no idle time slots available to switch to other carrier frequencies, which creates a problem when considering how to determine whether handover to a given base station on a given frequency is appropriate at a particular instant. Since the mobile station cannot provide any inter-frequency measurements to a handover evaluation algorithm operating either in the network or the mobile station, the handover decision will be made without full knowledge of the interference situation experienced by the mobile station, and therefore can be unreliable.
One possible solution to this problem is the provision of an additional receiver in the mobile unit which can be used to take measurements on candidate frequencies. Another possibility is to use a wideband receiver which is capable of simultaneously receiving and demodulating several carrier frequencies. However, these solutions add complexity and expense to the mobile unit.
Another solution is presented in U.S. Pat. No. 5,533,014 to Willars et al.,the disclosure of which is expressly incorporated here by reference, wherein this problem is addressed by introducing discontinuous transmission into CDMA communications techniques. In this patent, for example, a compressed transmission mode is provided using a lower spreading ratio (i.e., by decreasing the number of chips per symbol) such that with a fixed chip rate the spread information only fills a part of a frame. This leaves part of each frame, referred to therein as an idle part, during which the receiver can perform other functions, such as the evaluation of candidate cells at other frequencies for purposes of handover.
Various other mechanisms available for creating an idle part within a CDMA frame (which technique is sometimes referred to as xe2x80x9cslotted modexe2x80x9d operation) are also known, e.g., U.S. Pat. No. 5,883,899 entitled xe2x80x9cCode Rate Reduced Compressed Mode DS-CDMAxe2x80x9d to E. Dahlman and U.S. patent application Ser. No. 08/636,648, entitled xe2x80x9cMulti-Code Compressed Mode DS-CDMA Systems and Methodsxe2x80x9d, to E. Dahlman and filed on Apr. 23, 1996, the disclosures of which are also incorporated here by reference. Slotted mode operation is illustrated conceptually in FIG. 1. Therein, a plurality of downlink (DL) frame transmissions are depicted, each having a duration of 10 ms in this example. During frame #4, an idle portion is created by doubling the transmission rate during the beginning and end portion of the frame, as represented by the two higher bars 10 and 12. Corresponding frames are also illustrated for the uplink (UL).
The usage of a slotted mode technique to perform, for example, measurements on other channels raises a problem, however, in connection with power control. Power control techniques are implemented in radiocommunication systems to ensure reliable reception of a signal at each remote station, i.e., to provide that the ratio of the signal to the interference (SIR) should be above a prescribed threshold for each remote station.
To improve the SIR for a remote station which drops below this threshold, the energy of the signal is increased to appropriate levels. However, increasing the energy associated with one remote station increases the interference associated with other nearby remote stations. As such, the radio communication system must strike a balance between the requirements of all remote stations sharing the same common channel. A steady state condition is reached when the SIR requirements for all remote stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each remote station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels raises interference experienced at each remote receiver, and limits the number of signals which may be successfully communicated on the common channel (e.g. reduces system capacity).
This technique for controlling transmit power in radiocommunication systems is commonly referred to as a fast power control loop. The initial SIR target is established based upon a desired quality of service (QoS) for a particular connection or service type. For non-orthogonal channels, the actual SIR values experienced by a particular remote station or base station can be expressed as:                     SIR        =                              Mean power of received signal                                Sum of the mean powers of all interfering signals                                              (        1        )            
The SIR is measured by the receiving party and is used for determining which power control command is sent to the transmitting party.
A slow power control loop can then be used to adjust the SIR target value on an ongoing basis. For example, the remote station can measure the quality of the signals received from the remote station using, for example, known bit error rate (BER) or frame error rate (FER) techniques. Based upon the received signal quality, which may fluctuate during the course of a connection between the base station and a remote station, the slow power control loop can adjust the SIR target that the fast power control loop uses to adjust the base station""s transmitted power. Similar techniques can be used to control uplink transmit power.
Applicant has recognized, however, that when employing slotted mode transmission in the downlink to permit remote stations to perform measurements, power control commands are not being transmitted to inform the remote stations of how to adjust their transmit powers for the uplink, e.g., during the time represented by the striped portion of frame #4 of the uplink in FIG. 1. This increases the likelihood of erroneous reception of information by the base station on the uplink due to improper transmit powers being used.
The impact of slotted mode transmissions on system capacity and performance has not been thoroughly investigated. It has previously been assumed that the slow power control loop will adequately handle power control during slotted mode transmissions, as well as normal mode transmissions.
However, using the slow power control loop to handle slotted mode transmission has the potential to cause another difficulty. Specifically, if slotted mode transmissions are made frequently, the BER (or FER) for that connection will increase. This, in turn, will cause the slow power control loop to adjust the SIR target, thereby increasing the transmit power in the uplink by an amount A as shown in FIG. 2. Therein, all of the frames are transmitted at a higher power than might otherwise be necessary absent the impact of slotted mode transmissions. However, in this situation where slotted mode transmissions are made rather frequently, using the slow power control loop to handle slotted mode transmissions suffers from the drawback that unnecessarily high power levels are used to transmit certain frames, e.g., at least some of frames #1-3 and #5-7 in FIG. 2, thereby effectively reducing capacity within the system. Alternatively, if slotted mode transmissions are made less frequently, the slow power control loop may provide little or no power adjustments, which may result in degraded BER/FER at the uplink receiver.
Accordingly, it would be desirable to provide a CDMA system in which transmission and reception was discontinuous (i.e., which employs slotted mode transmissions) but which avoids the aforementioned power control problems.
These and other problems, drawbacks and limitations of conventional CDMA techniques are overcome according to the present invention, wherein, according to a first exemplary embodiment, when entering slotted mode the transmit power in the uplink is increased based on an estimated fading margin. This improves performance during the idle period in the downlink when no power control commands can be transmitted to the remote stations. At the end of the idle period, the power control loop can then return the remote stations to optimal transmit power levels.
According to a second exemplary embodiment of the present invention, when entering slotted mode in one link, e.g., the downlink, slotted mode can also be entered in the other link, e.g., the uplink. In this way, transmissions are not performed on the uplink without power control information during the idle period on the downlink.